1. Field of the Invention
The present invention relates to a dielectric ceramics, a multilayer ceramic capacitor using the same and a method for manufacturing them.
2. Description of Related Art
In recent years, as mobile devices such as cellular phones are more widely used and semiconductor devices that are the main part of a personal computer have higher speed and higher frequency, a multilayer ceramic capacitor for these electronics is increasingly required to have small size and large capacity.
To meet such requirement, in a multilayer ceramic capacitor (MLC), efforts have been made for smaller size and larger capacity, by making dielectric layers thinner to enhance electrostatic capacity and increasing the number of lamination. Therefore, to meet the demand for thinner multilayered dielectric layers as above, in a dielectric layer constituting a multilayer ceramic capacitor, attempts have been made for atomization and better relative dielectric constant in dielectric powder constituting a dielectric ceramics (For example, Japanese Unexamined Patent Publication No. 2004-210636).
For example, the above publication mentions that barium titanate powder that is typical dielectric powder is obtained in the form of fine grains by mixing barium hydroxide aqueous solution and Ti alkoxide solution and ripening the mixture in a vessel for a predetermined time followed by dehydration and drying.
However, barium titanate powder obtained through the above-mentioned liquid phase method is dried under the condition of 110° C. and three hours after mixing and ripening. This condition is employed to only remove moisture contained in the mixture. The barium titanate powder has much crystal water and impurities such as hydroxide. The barium titanate powder so obtained is made so small as to have a mean grain size of 0.05 μm (50 nm), while having a lattice constant larger than the value figured out from a single crystal (a=0.4032 nm, V=0.065548 nm3) and, in terms of crystal structure, mainly having cubic crystals. Therefore, there has been a problem that a dielectric ceramics prepared by using the dielectric powder so obtained has a low relative dielectric constant.
Meanwhile, grains have been made finer to produce a flat dielectric layer that can meet the demand for thinner layers and to prevent reliability from being lowered by increasing an applied electric field to a multilayer ceramic capacitor due to thinner layers. For example, Japanese Unexamined Patent Publication No. 2003-309036 mentions that a dielectric layer is formed so that the thickness of dielectric layer t and the maximum diameter of glass grain D can satisfy the relation of D/t≦0.5, thereby attaining high insulation and improving reliability in high temperature load test. In addition, Japanese Unexamined Patent Publication No. 2003-40671 describes thinner dielectric layers and the use of barium titanate powder having a mean grain size of 0.4 μm to prevent a decrease in relative dielectric constant that occurs when applying DC bias.
According to Ferroelectrics, 1998, Vols. 206-207, pp 337-353, M. H. FREY, Z. XU, P. HAN and D. A. PAYNE, for example, barium titanate used mainly for a dielectric material of the above-mentioned multilayer ceramic capacitor has perovskite crystal structure, and it is known to show a very high relative dielectric constant of about 4800.
However, if fine barium titanate powder according to Japanese Unexamined Patent Publication No. 2003-309036, for example, is used to make dielectric layers thinner in manufacturing a multilayer ceramic capacitor, abnormal grain growth occurs during firing at atmospheric pressure. Therefore, crystal grains constituting a dielectric layer cannot have uniform grain size, and large crystal grains resulting from grain growth are present in part. There has been a problem that a multilayer ceramic capacitor having such crystal grains has larger temperature characteristic of relative dielectric constant and lower insulation, in particular, lower reliability in high temperature load test.